Braden Myers and Faculty Mentor: Jaron Hansen, Department of Chemistry and Biochemistry
The aim of this research was to demonstrate Caldicellulosiruptor bescii is a feasible microbial pretreatment of lignocellulose for enhanced production of biogas when coupled with secondary anaerobic digestion. Before anaerobic digestion of waste can reach its maximum energy production potential, economically sound methods for pretreatments that allow increased digestion of waste materials must be developed. Many types of waste cannot be broken down sufficiently by anaerobic digestion alone. Overcoming this hurdle through successful feedstock pretreatment would unlock a vast potential supply of renewable energy feedstock materials.
Caldicellulosiruptor bescii is an anaerobic bacterium found in Russia. The bacteria were cultured under anaerobic conditions at 75°C. Pretreatment of substrate was performed in 100 mL flasks under anaerobic conditions at 75°C at a concentration of 5 grams of substrate per liter of nutrient solution and inoculated with one mL of c. bescii bacteria culture. After the pretreatment with the c. bescii, the pretreated substrate was treated with acetogens and methanogens (used for biogas production). Biogas production was carried out in an 8-cell Tam-Air calorimeter. Heat liberated was measured using the calorimeter and pressure transducers allowed for real time measurement of the amount of gas produced (shown in figure 1). In addition to the pretreated substrate, untreated substrate was also digested using acetogens and methanogens, allowing comparison of the amount of heat liberated by the biogas production of both the pretreated and untreated substrate.
When a biological process is carried out, some energy is released as heat. In the context of this experiment, more heat means more growth, and since methane is one of the byproducts of methanogen growth, more methane produced. Measuring the heat liberated throughout the digestion will allow us to compare the relative digestibility before and after pretreatment. One concern that has been expressed is that the heat liberation may not always correspond to the biogas production step in the digestion. The ability to measure the amount of gas produced has provided us with the ability to better determine the digestibility of pretreated substrates.
The results show that c. bescii works to break down the substrate making it more suitable for digestion by the acetogens and methanogens and in turn better suitable for biogas production.
Previous experiments have shown that over time efficiency of the pretreatment decreased over time. This presented a major problem for large-scale use of this technology. If the pretreatment becomes less efficient, cost would increase due to having to replace the bacteria, making this process less feasible. After redesigning the process used for pretreatment and taking more care to avoid contamination, we have been able to show that there is no loss in the efficiency of the bacteria used in pretreatment over time. This finding has helped to show that this process could be feasible for use on a large, commercial scale.
Previous work on this project focused on more reliable and efficient ways of evaluating different pretreatment methods. One of the accomplishments of this project was that these methods were evaluated. Previously, heat liberated was the only measurement available to us for evaluation of pretreatment. Work performed on this project has now shown that while heat is an acceptable measurement of bacterial growth, pressure data allows for direct measurement of biogas production. This direct measurement allows for even better evaluation of c. bescii in the pretreatment of plant material. The next step is to identify the ratio of methane to carbon dioxide. Typically in anaerobic digestion, methane and carbon dioxide are produced in approximately a one to one ratio. Infrared spectroscopy has been proposed as a way to find the percentage of useable methane in the biogas.
A second accomplishment from the work on this that we have began to show that the pretreatment greatly improves the efficiency at which the acetogens and methanogens digest plant biomass. In some cases, the pretreatment increased the amount of gas produced by 10 times. This makes the process of anaerobic digestion of plant material much more efficient. These trials have shown that about 0.35 cubic meters of biogas can be produced per kilogram of pretreated plant material compared to about 0.03 cubic meters per kilogram for untreated material. While this is not as much gas per kilogram of other types of waste, plant waste typically does not produce much biogas and any improvement will make this process much more economical.
Over the last year, we have made progress in showing the improvements that can be made with the use of pretreatment. The increase in gas production will more than offset the increased cost of pretreatment. The data collected so far will provide the initial data required to proceed with this project, specifically building a working scale model.